The origin of enhanced photoelectrochemical activity in metal-ion-doped ZnO/CdS quantum dots

Abstract To improve the performance of photoelectrochemical cells, various metal ions have been incorporated into different host semiconductor nanocrystals. However, such an approach raises two new questions that must be answered to design and optimize this new doping strategy: (i) How do we accurately evaluate and rank these dopants? and (ii) how do the dopant-related midgap states tailor the band structure of the host semiconductors and the dynamics of the photogenerated carriers? In the present work, we use nanosheet-nanorod ZnO as a free-standing substrate and CdS as a host for indium, copper, manganese, and cerium doping. Given the uniform physical properties and electronic structure of this environment, the photocurrent density and photoelectrochemical efficiency increase from ZnO–CdS (3.0 mA/cm2, 0.41%), ZnO–CdS–In (4.4 mA/cm2, 0.59%), ZnO–CdS–Cu (5.1 mA/cm2, 0.98%), ZnO–CdS–Mn (6.7 mA/cm2, 1.07%), to ZnO–CdS–Ce (8.9 mA/cm2, 1.9%). With the help of electrochemical cyclic voltammetry, open-circuit voltage-decay measurements, and fluorescence spectroscopy, the band edges tailored by the midgap states and the carrier dynamics are clearly determined. Furthermore, a complex relationship between these properties and the material performance is established for accurately deciphering the photoelectrochemical activity. By monitoring the ·OH concentration during irradiation, the dynamic photoelectrochemical catalytic activity is also clearly depicted by using ZnO–CdS–Mn and ZnO–CdS–Ce as model photocatalysts.